Integrated device for capacitive measuring of nanometer distances

ABSTRACT

Within an integrated device for capacitive measuring of nanometer distances above a well formed in a substrate, one below the other, there are situated electrically conductive and electrically insulated plates, namely, a movable plate connected to the substrate, a sensing plate connected to an input of a follower amplifier, and a plate building up the electric field and being electrically insulated from the well and connected to a pulsing generator. The well is connected to the output of the follower amplifier and projects from below the sensing plate to the extent that the capacitance of the sensing plate with respect to the substrate is reduced to a minimum. At the drain of an input transistor within the follower amplifier a potential is maintained, which potential is changing in the same way as a potential at the output of the follower amplifier. The device of the invention makes possible an absolute measurement of distances. By an appropriate topology of the plates of the micromechanical sensor the parasitic capacitance of this sensor is eliminated, by means of an active circuit. The effect of parasitic capacitances in the pertaining electronic circuit is also eliminated.

The invention concerns an integrated device for capacitive measuring ofnanometer distances which makes it possible to measure absolutedistances being of order of magnitude of one nanometer.

The technical problem to be solved by the present invention is how toremove an influence of parasitic capacitances in a micromechanicalsensor itself as well as in a pertaining electronic circuit of anintegrated device for measuring nanometer distances so that an absolutemeasurement of such distances would be feasible.

Devices for capacitive measuring of nanometer distances comprise amicromechanical sensor and an electronic circuit producing input signalsfor the micromechanical sensor and processing output signals of thesensor.

Hitherto developed devices for capacitive measuring of nanometerdistances are fabricated mainly by a hybrid technique. They distinguishthemselves in that their micromechanical sensor comprises a movableplate of several square millimeters and in that their measuringcapacitance is even from 5 pF to 30 pF (Sensors and Actuators A, 39(1993) 209-217). With these devices good results are obtained. Themovable plate in the micromechanical sensor, however, is fabricated by atechnology differing from the technology for producing of the pertainingelectronic circuit. This is reflected in a high price of such devices.

Recently, some devices for capacitive measuring of nanometer distanceshave appeared in which also the movable plate of the micromechanicalsensor is integrated together with the pertaining electronic circuit. Insupplementary technological steps the micromechanical sensor isfabricated above the rest of the device whereat these supplementarysteps do not represent any substantial change in the technology ofmanufacturing the integrated circuit. Therefore these devices areconsiderably cheaper than the afore-described ones. The measuringcapacitance of the sensor, however, is only from 0.1 pF to 1 pF.Therefore the features of these devices are not so outstanding as thoseof the hybrid devices, nevertheless they are good enough for anapplication in numerous measuring systems. With respect to the pick-upof the signals the following two embodiments are used.

Very common is an embodiment based on a differential distancemeasurement. A movable plate of a micromechanical sensor is fastenedbetween two fixed plates and with them it forms two capacitors (AnalogDevices ADXL50). They are connected to signal generator outputs of anopposite phase. When the movable plate is situated in the middle betweenthe fixed plates the signals cancel each other. The sensor output isconnected to a synchronized demodulator. A disadvantage of thisembodiment is an exacting fabrication of the movable plate and of thetwo fixed plates. Moreover, the central movable plate must be insulatedwith respect to the substrate since it is connected to the input of theelectronic circuit in the device.

Further, also an embodiment having two micromechanical sensors connectedto a bridge has been known (Proc. IEEE Solid-State Sensor and ActuatorWorkshop, pp. 126-131, June 1992). Also in this embodiment the movablesensor plates must be insulated with respect to the substrate.

A common disadvantage of all integrated devices for capacitive measuringof nanometer distances exists in that parasitic capacitances of thesensor plates with respect to integrated circuit layers below arestrongly pronounced. The sensor output signal is additionally reduced bythe parasitic input capacitance of the pertaining electronic circuit.

Known integrated devices for capacitive measuring of nanometer distancesdo not render possible an absolute distance measurement. The influenceof parasitic capacitances therein is not sufficiently reduced that by asignal at the output of the micromechanical sensor the distance betweenthe movable plate and the sensing plate of the sensor could bedetermined. However, since knowing this distance is crucial for anevaluation of the sensor sensitivity, known integrated devices forcapacitive measuring of nanometer distances need to be calibratedmechanically.

The said technical problem is solved by an integrated device of theinvention for capacitive measuring of nanometer distances, which deviceis characterized in that above a well in a substrate, one below theother there are situated electrically conductive and electricallyinsulated plates, namely, a movable plate, a sensing plate and a platebuilding up the electric field and being electrically insulated from thewell, and the well projects from below the sensing plate to the extentthat the capacitance of the sensing plate with respect to the substrateis reduced to a minimum. The plate building up the electric field isconnected to a pulsing generator and the sensing plate is connected tothe input of a follower amplifier and the well is connected to theoutput of the follower amplifier. The movable plate is connected to thesubstrate. At the drain of an input transistor within the followeramplifier a potential is maintained, changing in the same way as apotential at the output of the follower amplifier.

The device of the invention is further characterized in that the platebuilding up the electric field is formed above a first oxide layer andthe sensing plate is formed in an upper metal layer of the integratedcircuit with the plate building up the electric field screening a largepart of the sensing plate.

The device of the invention is further characterized in that thepotential at the input of the follower amplifier within a first timeslot of each cycle equals a potential of the sensing plate and within asecond time slot of each cycle on the sensing plate a potential is builtup which causes the sensing plate to attract the movable plate so thatthe latter is moved back to the sensing plate.

And finally, the device of the invention is characterized in that withinthe first time slot after the expiration of the clock signal a potentialat a substrate of two switching transistors, whose drain and source,respectively, are connected to the sensing plate and which are providedfor switching between the first time slot and the second time slot, andthe potentials on the gates of the two transistors are raised parallellywith respect to potential at the output of the follower amplifier.

The most important advantage of the integrated device of the inventionfor capacitive measuring of nanometer distances with respect to otherdevices of this type exists in that it makes possible an absolutemeasurement of distances. By choosing an appropriate topology theparasitic capacitance of the micromechanical sensor is removed in asimple way and by an active circuit the influence of parasiticcapacitances in the pertaining electronic circuit is eliminated. Thedevice of the invention distinguishes itself also by a very simplyrealized attachment of the movable plate to the device substrate.

The invention will now be described by way of example and with referenceto the accompanying drawings representing in:

FIG. 1 a circuit of an integrated device of the invention for capacitivemeasuring of nanometer distances,

FIG. 2 a section across a micromechanical sensor in the device of theinvention,

FIG. 3 the time dependence of several potentials in the device of theinvention.

The integrated device of the invention for capacitive measuring ofnanometer distances comprises a micromechanical sensor 1, a followeramplifier 4 and several components, e.g. transistors 31, 32, a voltagedoubler 34 and a negator 33 (FIG. 1), which are intended to improve theoperation of the device according to the invention. To the deviceaccording to the invention also several signals are fed which will bedescribed in the following.

The micromechanical sensor 1 comprises a movable plate 11, a sensingplate 12, a plate 13 building up the electric field and a well 14. Theseelements with the exception of the movable plate 11 are fabricated by astandard process of manufacturing an integrated circuit on a substrate15 (the description relates to a p-type substrate) and they areelectrically insulated with respect to each other (FIG. 2). The plate 13building up the electric field is fabricated to cover a limited area ontop of a first oxide layer 17. The plate 13 building up the electricfield is situated on top of the well 14, the surface of the plate 13being smaller than the surface of the sensing plate 12. In a secondmetal layer on top of a second oxide layer 16 the sensing plate 12 isfabricated which is situated above the plate 13 building up the electricfield and, evidently, also above the well 14 in such a manner that thewell 14 everywhere projects a little from below the sensing plate 12.The movable plate 11 is fabricated as a beam which is on one sidefastened to the substrate 15 and the upper part of which is the movableplate 11 extending above the sensing plate 12.

Hence the movable plate 11 is electrically connected to the substrate15. The plate 13 building up the electric field is connected to theoutput of a pulsing generator 2. The sensing plate 12, however, isconnected to the gate of an input transistor 41 within the followeramplifier 4, whose output o is electrically connected to the well 14.

The follower amplifier 4 is made as follows. The drain of the inputtransistor 41 is connected through cascade-connected transistors 42 and43 to the output o of the follower amplifier 4. A terminal common to thegates of the transistor 42 and of a transistor 42′ connectedsymmetrically to the transistor 42 and to the gate and the drain of thetransistor 43 is connected through a current generator 441 to a lowsupply voltage terminal. Within the follower amplifier 4 its dominant RCelement 45, an output transistor 46 and a transistor arrangement 44 areinterconnected in a known way, the transistor arrangement 44 providingfor a parallel variation of the potentials at the sources of thetransistors 42 and 42′. The output o of the follower amplifier 4 isconnected to the gate of a transistor 41′ being connected symmetricallyto the transistor 41. The emitters of the transistors 41, 41′ areconnected through the current source 411 to a high supply voltageterminal. The high supply voltage is conducted to the output of thefollower amplifier 4 on the one hand through a current generator 47 andon the other hand through a series connected current generator 481, acontrolled switch 48 and a transistor 49.

To the sensing plate 12 an operation point potential Vop is conductedthrough a transistor 31. The gate of the transistor 31 is connectedthrough a negator 33, which is composed of transistors 331 and 332 andwhich is controlled by a clock signal Vc, to the common terminal of aswitch 48 and of a current source 481. To this common terminal also thesubstrates of the transistors 31 and 32 and the control input of avoltage doubler 34 are connected. To the control input of the voltagedoubler 34 a potential Vm is supplied which determines when the distancemeasurement is carried out and when a restoring force is exerted on themovable plate 11. By the potential Vm also the pulsing generator 2 andthe switch 48 are controlled. The output of the voltage doubler 34 isconnected to the gate of the transistor 32, to whose drain a potentialVr provided for the exertion of the restoring force on the movable plate11 is connected.

The integrated device of the invention for capacitive measuring ofnanometer distances actually operates as a capacitive voltage divider.This voltage divider consists of two capacitors. They are fabricated byusing several integrated circuit levels. The first capacitor is areference capacitor having a capacitance Cr with the plate 13 buildingup the electric field and the sensing plate 12 representing its plates,the second capacitor is a sensing capacitor having a capacitance Cs withthe sensing plate 12 and the movable plate 11 representing its plates(FIG. 2). To the plate 13 building up the electric field square-wavepulses Vp of a known amplitude are conducted (FIG. 3). On the sensingplate 12 this amplitude is reduced by a factor Cr/(Cr+Cs) thecapacitance Cs depending on the current position of the movable plate11.

The amplitude of the potential V12 on the sensing plate 12 and,consequently, also of the potential Vo at the output o of the followeramplifier 4, however, would be much smaller if parasitic capacitances inthe device of the invention or their effect had not been eliminated.

The parasitic capacitance between the sensing plate 12 and the substrate15 is eliminated by the plate 13 building up the electric fieldscreening a large part of the sensing plate 12 and especially by thewell 14 connected to the output of the follower amplifier 4 beingfabricated in the substrate 15 below the sensing plate 12. The well 14projects from below the sensing plate 12 to such an extent that theparasitic capacitance of the sensing plate 12 with respect the substrate15 is reduced as much as possible.

The effect of parasitic capacitances between the terminals of thetransistors 41, 31 and 32, which are connected to the sensing plate 12,however, is eliminated by correctly chosen time dependences ofpotentials at these terminals. In a manner already mentioned, by meansof the cascade connected transistors 42, 43 at the drain of the inputtransistor 41 a potential V41 d is built up changing in the same way asthe potential Vo and therefore changing also in the same way as apotential at the gate of this transistor and therefore matching apotential V41 s at the substrate of this transistor 41 (FIG. 3). Thusthe effect of parasitic capacitances between the drain and the gate orthe substrate of the input transistor 41 is eliminated.

Also to the gate and to the substrate of the transistor 31 a potentialVs is conducted, whose level is shifted with respect to the potential Voat the output of the follower amplifier 4. The potential Vo coincideswith the potential V12 at the gate of this transistor. In this way theinfluence of the parasitic capacitance between the drain and the gate orthe substrate of the transistor 31 is eliminated. Similarly also theeffect of the parasitic capacitances between the drain and the gate orthe substrate of the transistor 32 are eliminated, the transistor 32,however, does not conduct when actually measuring a distance.

In the manner described above the actual total parasitic capacitance inthe device of the invention is reduced below 2 fF, which makes itpossible that by means of this device the distance between the movableplate 11 and the sensing plate 12 of the micromechanical sensor 1 can bemeasured absolutely.

The operation of the device of the invention described so far refers tothe measuring phase of the device operation. However, the deviceoperation is actually composed of time cycles, whereat in each cycle afirst time slot S from t=0 to t=t2 is provided to perform themeasurement of the distance between the movable plate 11 and the sensingplate 12 of the micromechanical sensor 1 and a second time slot R fromt=t2 to t=t3 is provided to set the initial position of the movableplate 11 of the sensor 1, i.e. to attract the movable plate 11 to thesensing plate 12. The phase of the operation of the device of theinvention is determined by the level of a control potential Vm (FIG. 3).During the measuring phase the potential Vm is at a high level, duringthe restoration phase, however, it is at a low level.

The control potential Vm changes to the high level whenever a pulse ofthe clock signal Vc appears (FIG. 3). The clock signal Vc stays at thehigh level so long—from t=0 to t=t1—that the sensing plate 12 of thevoltage divider acquires a proper operation potential Vop. The pulsinggenerator 2 controlled by the signal Vm delivers a pulse Vp when theclock signal Vc appears.

In the first time slot S of each cycle, i.e. in the measuring phase ofthe device operation, the potential at the gate of the input transistor41 within the follower amplifier 4 equals the potential V12 of thesensing plate 12. During the measuring phase the switch 48 controlled bythe signal Vm is closed and at the potential Vs there are the substratesof the transistors 31, 32 as well the control input of the voltagedoubler 34 through which actually the potential Vs is conducted to thegate of the transistor 32. When in the moment t1 the clock signal Vcjumps to the low level, the negator 33 passes the potential Vs also tothe gate of the transistor 31. In this way in the main part of themeasuring phase i.e. from t=t1 to t=t2, the effect of the parasiticcapacitances of the transistors 31 and 32 is eliminated.

In the second time slot R of each cycle, i.e. in the restoration phase,on the sensing plate 12 such a potential is built up that the movableplate 11 is attracted to the sensing plate 12. The application of thesensing plate 12 also for performing the restoration of the movableplate 11 back to the sensing plate 12 is made possible by the transistor32 being controlled by a voltage doubler 34. In the restoration phasefrom t=t2 to t=t3 the controlled switch 48 is open. At the controlterminal of the voltage doubler 34 a high supply potential isestablished. At the transition from the measuring phase S to therestoration phase R the potential at the gate of the transistor 32 ischanged from Vs, when the transistor 32 is closed, to the doubledpotential of the low supply. The transistor 32 is opened completely andthe movable plate 11 moves to the initial position.

Obviously the device of the invention for capacitive measuring ofnanometer distances is also suitable for the measuring acceleration.

What is claimed is:
 1. Integrated device for capacitive measuring of nanometer distances comprising a well (14) formed in a substrate (15), a plurality of electrically conductive plates situated above said well, and overlying each other, including a movable plate (11), a sensing plate (12) situated below said movable plate, and a plate (13) situated below said sensing plate building up an electric field, said plates (11, 12, 13) being electrically insulated from each other and from the well (14) wherein said well (14) projects from below the sensing plate (12) to such an extent that the capacitance of the sensing plate (12) with respect to the substrate (15) is reduced to a minimum, wherein said plate (13) building up the electric field is connected to a pulsing generator (2), and said sensing plate (12) is connected to the input of a follower amplifier (4), said well (14) is connected to the output (o) of the follower amplifier (4) and the movable plate (11) is connected to the substrate (15), said follower amplifier (4) including an input transistor (41) having a drain at which is maintained a potential (V41 d) which changes in the same way as a potential (Vo) at the output of the follower amplifier (4).
 2. Integrated device for capacitive measuring of nanometer distances as recited in claim 1, wherein said plate (13) building up the electric field is formed above a first oxide layer (17) and the sensing plate (12) is formed in an upper metal layer of the integrated device, wherein said plate (13) building up the electric field screens a large part of the sensing plate (12).
 3. Integrated device for capacitive measuring of nanometer distances as recited in claim 2, wherein said potential at the input of the follower amplifier (4) within a first time slot (S) of each cycle is equal to a potential (V12) of the sensing plate (12), and within a second time slot (S) of each cycle, a potential is built up on said sensing plate (12) which causes the sensing plate (12) to attract the movable plate (11).
 4. Integrated device for capacitive measuring of nanometer distances as recited in claim 3, wherein within the first time slot (S) after expiration of a clock signal (Vc) at two switching transistors (31, 32), whose drain and source, respectively, are connected to said sensing plate (12) and which are provided for switching between the first time slot (S) and the second time slot (R), a potential (Vs) on the substrate of the transistors (31, 32) and potentials (V31 g, V32 g) on the gates of the transistors (31, 32) are raised parallelly with respect to a potential (V0) at the output (o) of the follower amplifier (4). 